Field of the Invention
The present invention relates to a thermal processing apparatus and a thermal processing method for heating a precision electronic substrate (hereinafter simply called a “substrate”) in the form of a thin plate such as a semiconductor wafer or a glass substrate for a liquid crystal display by irradiating the substrate with light.
Description of the Background Art
Impurity introduction performed to form a pn junction in a semiconductor wafer is an essential step in manufacturing process of a semiconductor device. At present, impurities are introduced generally by ion implantation process and subsequent annealing process. The ion implantation process is a technique of implanting impurities physically by ionizing an impurity element such as boron (B), arsenic (As) or phosphorous (P) and making the impurity ions collide with the semiconductor wafer at a highly accelerated voltage. The implanted impurities are activated by the annealing process. If the annealing takes about several seconds or more, the implanted impurities are diffused deeply by heat and a resultant junction reaches a depth greater than is necessary. This might become an obstacle to favorable formation of the device.
Flash lamp annealing (FLA) has attracted attention in recent years as an annealing technique of heating a semiconductor wafer within an extremely short time. The flash lamp annealing is a thermal processing technique of increasing the temperature only of a front surface with implanted impurities of a semiconductor wafer within an extremely short time by irradiating the front surface of the semiconductor wafer with a flash using a xenon flash lamp (in the below, a lamp simply called a “flash lamp” means a xenon flash lamp).
The spectral distribution of light emitted from a xenon flash lamp ranges from an ultraviolet region to a near-infrared region, has a shorter wavelength than light from a conventional halogen lamp, and substantially agrees with the base absorption band of a silicon semiconductor wafer. Thus, irradiating a semiconductor wafer with a flash from the xenon flash lamp does not produce much transmitted light so that the temperature of the semiconductor wafer can be increased rapidly. Additionally, it has become known that irradiation with a flash within an extremely short time of several milliseconds or less can increase the temperature only of a front surface and its vicinity of the semiconductor wafer selectively. As a result, increasing a temperature within an extremely short time with the xenon flash lamp can realize only activation of impurities without causing deep diffusion of the impurities.
US 2009/0175605 discloses a thermal processing apparatus using such a xenon flash lamp. In this thermal processing apparatus, the flash lamp and a halogen lamp are arranged on the sides of a front surface and a back surface respectively of a semiconductor wafer, and desirable thermal processing is realized by a combination of these lamps. In the thermal processing apparatus of US 2009/0175605, the semiconductor wafer held on a susceptor is preheated to a certain temperature with the halogen lamp, and then is heated to a desirable processing temperature by being irradiated with a flash from the flash lamp.
Regarding heating of a semiconductor wafer by light irradiation, what is important is to make an illumination distribution uniform to achieve better uniformity of a temperature distribution in the plane of a wafer, regardless of whether a light source is a flash lamp or a halogen lamp. Meanwhile, in a flash lamp annealing apparatus such as that of US 2009/0175605, a flash lamp and a halogen lamp are arranged above and below a chamber respectively, and the front and the back of a semiconductor wafer held on a susceptor are irradiated with light. Hence, even if a uniform illumination distribution in the plane of the wafer is achieved during irradiation of light from the halogen lamp, for example, the illumination distribution may become nonuniform during irradiation of a flash from the flash lamp. This has made it difficult to achieve a uniform temperature distribution during both irradiation of light from the flash lamp and irradiation of light from the halogen lamp.